No. r,72] 



L1TERATUBE 



501) 



way the pairs an- turned with reference to each other. The first 

 alignment gives gametes A A 1 and aa 1 , the second gives Aa 1 and 

 aA 1 . Thus three gametes with a dominant factor to one pure 

 recessive would on the average he produced, the ratio heing 

 1AA : 2Aa : laa, omitting primes. 



In a tetraploid form, however, A and A 1 are alike and inter- 

 changeable, as also are a and a*. In the hybrid — \, therefore, 

 there would be at least one other mode of pairing of alleh .nrni-phs 

 possible, giving two new modes of line-up on the reduction 

 spindle, and they would occur just as frequently as the two 

 previous kinds. The two new arrangements would be^ — , 

 giving gametes AA 1 and a*a, like those in the first of the two 

 previous cases, and ~ ^j, giving gametes Aa and a 1 A 1 . 8 These 

 latter gametes would be indistinguishable from the Aa 1 and aA 1 

 gametes given by the second of the two usual arrangements un- 

 less A could be distinguished from A 1 and a from a 1 . This could 

 happen only if the allelomorphs were of four different kinds or 

 if there were linkage of these genes with other genes for which 

 the plant was heterozygous. Unless, therefore, linkage or mul- 

 tiple allelomorphism were involved, we could not distinguish 

 between this mode of pairing of allelomorphs and the usual kind : 

 both would give three gametes containing at least one dominant, 

 to one pure recessive (»'. c, lAA:2Aa:laa, omitting primes). 



Still a third type of pairing of allelomorphs is possible in a 

 tetraploid plant, however. There seems no a priori reason, on 

 the chromosome view, why, in a tetraploid plant, a gene should 

 have to segregate from one of the allelomorphs derived from the 

 opposite parent. That is, in a plant of composition — -j- , 

 paternally derived genes being indicated on. say. the upper line, 

 maternally derived ones on the lower, there is no apparent reason 

 why the line-up of chromosomes at reduction should not be 

 Ti~ or as often as it is one of the other types, since all four 

 chromosomes are homologous. Thus we should get gametes Aa, 

 A J a\ Aa 1 and A x a. 



